The goal of this project is to develop new control systems to restore standing function and enhance the postural stability of individuals paralyzed by spinal cord injuries (SCI). Systems that provide the ability to stand, alter standing posture, and maintain balance by automatically adjusting stimulation to the paralyzed muscles will be designed, optimized in simulation, and evaluated experimentally in six volunteers with SCI. The project will result in a unique, comprehensive balance control system that extends the capabilities and improves the safety of all currently available standing neuroprostheses. The first aim is to design, implement and test posture-follower and regional set-point control sub- systems. The posture-follower control element will automatically alter stimulation as users vary their standing postures about the nominal erect position by simply pulling or pushing against a walker. This will ensure that the optimal stimulation to support the body is applied continuously as the center of mass is smoothly relocated to a new location. The regional set-point control element will automatically adjust stimulation to resist disturbances and maintain balance based on joint angle position and center of mass acceleration. This sub-system will be optimized to span the entire base of support and sustain the desired posture defined by the posture-follower. These new control elements will be designed and evaluated individually in simulation, followed by laboratory demonstration and clinical assessment in volunteers with SCI. The sub-systems will then be integrated and compared to constant activation of the paralyzed muscles. The resulting controller should facilitate standing reach and other functional activities of daily living, require less upper extremity effort to maintain balance, resist larger applied perturbations, and be perceived as easier to use than conventional methods of standing. The second specific aim is to develop the capability to execute a reactive step. This new control element will automatically change foot position to expand the base of support sufficiently to remain standing in response to large, destabilizing disturbances. Work will begin by fully characterizing the electrically-induced flexion withdrawal reflex and evaluating its potential for generating a rapid change in foot placement. These data will be incorporated into computer simulations to identify an appropriate trigger and optimize patterns of stimulation to generate reproducible stepping motion. The resulting sub-system will take action if the applied perturbations exceed those effectively resisted by the set-point controller, and thus avoid impending falls. Effectiveness will be fully assessed in simulation and laboratory experiments involving application of repeatable external perturbations. Finally, all three sub-systems will be integrated into a comprehensive balance control system and thoroughly assessed with recipients of advanced surgically-implanted 16-channel stimulators.